The reduction of bisulfite to hydrogen sulfide, by sulfate-reducing bacteria, proceeds through a pathway involving trithionate and thiosulfate as intermediate compounds. We recently established that a reddish-brown pigment (P582), isolated from Desulfotomaculum nigrificans, catalyzed the reduction of bisulfite to trithionate. The enzyme catalyzing the next step, i.e., reduction of trithionate to thiosulfate has not been described. We proposed to purify and isolate an enzyme from D. nigrificans which carries out this reductive process. We plan to extend these studies to Desulfovibrio vulgaris and determine if the two organisms reduce trithionate to thiosulfate with similar enzymes.